What is positive displacement flow meter?
Positive displacement (PD) flow meters are flow meters that provide high accuracy (±0.1% of actual flow in some cases) and good repeatability (up to 0.05% of reading). These meters require no power supply to operate and no direct upstream or downstream piping for installation. They are well suited for flow measurement of viscous fluids, such as oils, fuels and solvents, which are difficult to measure with other types of flow meters.
PD flow meters are most widely used in home water meters, with millions of these units produced each year. Volumetric flow meters can be used for clean, sanitary and corrosive fluids such as water and food, as well as some gases. They are often most applicable when high accuracy at a reasonable price is required.
How does a positive displacement flow meter work?
Positive displacement flow meter technology is the only flow measurement technology that directly measures the volume of fluid passing through the meter. Volumetric flowmeters do this by repeatedly intercepting fluid to measure its flow rate. This process can be thought of as repeatedly filling a bucket with fluid before pouring the contents downstream. The number of times the bucket is filled and emptied indicates the flow rate through the flowmeter. There are many volumetric flow meter geometries available.
Retention is typically achieved using rotating components that form a moving seal between each other and/or the flowmeter body. In most designs, the rotating parts have tight tolerances so that these seals prevent fluid from passing through the flowmeter without being measured (sloshing). In some volumetric flowmeter designs, bearings are used to support the rotating parts. Rotation can be sensed mechanically or by detecting the motion of the rotating parts. As more fluid flows, the rotating parts rotate proportionally faster. The transmitter processes the signal generated by the rotation to determine the flow rate of the fluid. Some volumetric flow meters have mechanical registers that show the total flow rate on a local display.
Suitable for smaller line sizes, low flow rates, high viscosities and long durations, especially for oils. The disadvantages are that there are moving parts to wear, maintenance required, more impurities, and not as updated as other technologies with new protocols, etc.
- Accuracy: One of the main benefits of using PD flow meters is the high accuracy they offer, the high accuracy of the internal components means keeping the clearances between the sealing surfaces to a minimum. The smaller these clearances are, the higher the accuracy associated with them. Only fluids that can bypass this seal are not accounted for, this is called “bypass” or “sloshing”.
- Range and repeatability: Another benefit is that flow meters can handle a wide range of viscosities, and it is not uncommon to experience higher levels of accuracy when dealing with high viscosity fluids, simply due to the reduction of bypasses. When considering and comparing flowmeter accuracy, it is important to understand “linearity”; i.e., the ability of the meter to accurately measure the entire regulation ratio, and “repeatability”, i.e., the ability to remain accurate over multiple cycles. This is another area where PD flow meters excel, with 0.02% repeatability and 0.5% linearity being standard.
- Reliability: If the right flow meter is selected for an application, it can be expected to operate properly for many years. Meters are often sent for repair and recalibration that have been in the field for 10 and sometimes 20 years of continuous use. This reliability is due in large part to the fact that the same proven technology has been in use for more than 60 years, allowing major advances to be focused on tribology and achieving the required accuracy at a reasonable cost.
- Low maintenance: The recommended maintenance level is strongly influenced by the application. For example, if the flow meter is handling a fluid with lubricating properties (i.e. oil), routine maintenance can be virtually eliminated. However, if the fluid has poor lubrication properties, then it is best to discuss maintenance requirements with your distributor.
It is very rare that any maintenance on a positive displacement flow meter is more frequent than other equipment within the same system and can be scheduled at the same time, thus minimizing downtime.
Types of displacement flow meters
The operation of a volumetric (PD) flow meter consists of separating the liquid into precisely measured increments and continuing to move. Each segment is counted by a connection register. Because each increment represents a discrete volume, positive displacement units are popular in automated batching and accounting applications. Volumetric flow meters are ideal for measuring the flow of viscous liquids or where a simple mechanical instrumentation system is required.
Positive displacement flow meters for liquids or chapter-action disc meters
Chapter-action disc meters are the most common type of PD meter. They are used as residential water meters around the world. As water flows through the metering chamber, it causes the disc to oscillate (chapter motion), which turns the spindle, which rotates the magnet. This magnet is coupled to a mechanical register or pulse transmitter. Because the meter captures a fixed amount of fluid each time the spindle rotates, the flow rate is proportional to the speed of rotation of the spindle.
Oscillating piston flowmeter
Oscillating piston flow meters are typically used in viscous fluid services, such as oil metering on engine test stands, where the regulation ratio is not critical. These meters are also used in residential water service and can pass a limited amount of dirt, such as pipe scale and fine sand (i.e. -200 mesh or -74 micron), but not large particles or abrasive solids.
Elliptical Gear and Convex Angle Gauges
The elliptical gear partial discharge meter uses two fine-tooth gears, one mounted horizontally and the other vertically, with the gears meshing at the tip of the vertical gear and the center of the horizontal gear. The two rotors rotate relative to each other, thereby creating interception in the crescent-shaped gap between the housing and the gears. These gauges can be very accurate if the slippage between the housing and gear is kept small. If the process fluid viscosity is greater than 10 centipoise and the flow rate is greater than 20% of rated capacity, an accuracy of 0.1% AR can be obtained. At lower flow rates and lower viscosities, slippage increases and accuracy decreases to 0.5% AR or less.
The Helical Meter is a volumetric device that uses two radially inclined helical gears to continuously trap the process fluid as it flows. The flow forces the helical gears to rotate in the plane of the pipe. Optical or magnetic sensors are used to encode a sequence of pulses proportional to the rotation speed of the helical gears. The force required to rotate the helix is relatively low, resulting in a relatively low pressure drop compared to other PD instruments. The best achievable accuracy is approximately ±0.2% or rate.
Metering pumps are PD instruments that also transfer kinetic energy to the process fluid. There are three basic designs: peristaltic, piston and diaphragm.
Peristaltic pumps run by systematically squeezing plastic tubing through a finger or cam into a housing, which is also used to position the tubing. These metering pumps are used in laboratories, various medical applications, most environmental sampling systems, and for dispensing hypochlorite solutions. The tubing can be silicone rubber or, if a more corrosion resistant material is required, PTFE tubing can be used.
The piston pump delivers a fixed volume of fluid on each “out” stroke and a fixed volume of fluid into the chamber on each “in” stroke. A check valve prevents fluid from flowing backwards. As with all positive displacement pumps, piston pumps generate pulsating flow. To minimize pulsation, multiple pistons or pulsation dampening vessels are installed. Because of the tight tolerances of the piston and cylinder liner, a flushing mechanism must be provided in abrasive applications. The size of the piston pump depends on the piston displacement as well as the required flow and discharge pressure. Select a check valve (or in critical applications, a double check valve) to prevent backflow.
Diaphragm metering pumps are the most common industrial PD pumps. A typical configuration includes a diaphragm, a chamber, and suction and discharge check valves to prevent backflow. The piston can be connected directly to the diaphragm or can force hydraulic fluid to actuate the diaphragm. Maximum output pressure is approximately 125 psig. variants include bellows diaphragms, hydraulically actuated double diaphragms, and pneumatically reciprocating double diaphragms.
Gas positive displacement flow meters
PD gas flow meters operate by calculating the cut-off volume of the gas passing through, similar to the way PD flow meters operate on liquids. The main difference is that the gas is compressible.
Diaphragm gas meters are most commonly used to measure the flow of natural gas, especially for household metered consumption. The meter is made of cast aluminum with a cloth-backed rubber diaphragm. The meter consists of four chambers: two diaphragm chambers on the inlet and outlet side and an inlet and outlet chamber in the meter body. The passage of the gas through the meter creates a pressure difference between the two diaphragm chambers through the diaphragm chamber on the compression inlet side and the diaphragm chamber on the expansion outlet side. This action alternately empties and fills the four chambers. A slide valve at the top of the instrument alternately acts on the chambers and synchronizes the action of the diaphragms, as well as the crank mechanism that operates the instrument register.
High Precision Positive Displacement Systems
A high accuracy gas meter is typically a hybrid that combines a standard positive displacement meter with a motor drive that eliminates pressure drop across the meter. Balancing inlet and outlet pressures eliminates slip, leakage and gas fugitive. In a high-precision gas meter installation, a high-sensitivity vane is used to detect differential pressure and a displacement sensor is used to measure vane deflection.
How to use volumetric flow meters?
Positive displacement flow meters measure the volumetric flow of fluids in pipelines, such as water, hydrocarbons, cryogenic liquids and chemicals. Some designs can measure gas flow, although liquid flow applications are more common. In liquid service, increasing viscosity reduces slip and increases the pressure drop through the flowmeter. Surprisingly, under low flow conditions for a given volumetric flow meter, accuracy can actually improve when viscosity increases and slip decreases.
Larger pressure drops across the flow meter can prematurely wear and/or damage bearings and/or seals. Therefore, most positive displacement flow meters have a maximum pressure drop specification designed to limit positive displacement flow meter bearing wear to a reasonable level. Operating the meter above the meter pressure drop limit can result in premature bearing wear and catastrophic meter failure. Note that the flowmeter size can be increased to reduce the pressure drop in these applications. This may significantly increase the cost, but failure to comply with this specification may be more expensive in some applications.
Be careful, as damaged sealing surfaces can increase slippage and reduce measurement accuracy. Using a positive displacement flowmeter in abrasive or dirty fluids can cause maintenance problems because of the potential for damage to sealing surfaces, damage to bearings, and/or clogging of the flowmeter. A filter may be required to remove dirt.
When using a volumetric flowmeter, ensure that air bubbles are removed from the fluid stream. Flow measurements made in the presence of air bubbles will be higher than the true liquid flow because the bubble volumes are measured as if they were a certain volume of liquid. Therefore, the presence of air bubbles and (especially) the presence of varying amounts of air bubbles can adversely affect the flow measurement associated with a volumetric flow meter. Gas eliminators may be required to remove air bubbles and mitigate this problem.
The flowmeter can be used for clean, sanitary, corrosive liquids such as water and food, as well as some gases. Materials of construction are important because small amounts of corrosion or wear can damage sealing surfaces and adversely affect measurement accuracy. In addition, all wetted parts should be considered, including the valve body, rotating parts, bearings and gaskets.
Municipal water districts use many volumetric flow meters to measure residential water usage. Considering the installed base of millions of houses and apartments with metered water service, this application probably represents one of the largest numbers of volumetric flowmeter applications worldwide.
Corrosive liquid applications are common in chemical industrial processes and chemical feed systems used in most industries. However, other flowmeter technologies may be more suitable for these services.
- Oil and gas
- Water and Wastewater
- Food and Beverage
- Pulp and Paper
- Metals and Mining
Application considerations for volumetric flow meters
Avoid using Positive displacement flowmeters in dirty fluids unless fouling can be effectively removed upstream of the flowmeter. Operating these flowmeters in dirty fluids can lead to clogging and increased maintenance costs. Be careful when selecting bearings, as the non-lubricated nature of certain fluids, impurities and dirt can increase bearing wear and maintenance costs. Note that bearings usually do not always fail catastrophically; they may slow down and adversely affect accuracy before they stop working.
Avoid using fluids with air bubbles unless they can be effectively removed. As viscosity increases, be sure that the pressure drop through the meter is acceptable. Make sure the viscosity of the working fluid is similar to the viscosity of the calibration fluid, as different fluids exhibit different amounts of slip that can lead to measurement errors.
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